1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device using an optical resonance effect.
2. Description of the Related Art
Electroluminescent (EL) display devices may be classified into inorganic and organic devices depending on the material composing their emitter layers.
In organic EL devices, externally injected electrons and holes combine in an emitter layer and dissipate to generate excitons. As these excitons transit from an excited state to a base state, energy is transferred to fluorescent molecules in the emitter layer, enabling the molecules to emit light and display images. For every three triplet excitons, only a singlet exciton is generated, the energy level of which is different from that of the triplet excitons. Energy from singlet excitons is used for luminescence, whereas energy from triplet excitons is converted into thermal energy.
Organic EL devices provide a greater brightness, a greater driving voltage, a faster response rate, a wider viewing angle, and a sharper contrast, as compared with inorganic EL devices, and advantageously may display a range of colors. Thus, organic EL devices have received considerable attention and are regarded as the next-generation of display devices.
A general organic EL device includes an anode layer formed as a predetermined pattern on a substrate, a hole transporting layer, an emissive layer, and an electron transporting layer, which are sequentially stacked upon one another, and a cathode layer formed as a predetermined pattern intersecting the pattern of the anode layer at a right angle. All of the hole transporting layer, the emissive layer, and the electron transporting layer are organic thin films formed of organic substances.
Conventional organic EL devices are disadvantageous, for example, in terms of luminance, power consumption, and lifetime, due to their low luminance efficiency. Especially for organic EL devices which contain low molecular weight fluorescent dyes, emission efficiency is very low due to a low maximum emission efficiency of 25% from singlet excitons, reduced external light emission caused by internal total reflection, destructive interference with light reflected from the cathode layer, and light absorption by a polarizing plane. Therefore, high-luminance, low-power display devices may not be realized with such conventional organic EL devices. Furthermore, conventional organic EL devices have wide photoluminescent spectra, so that color purity is reduced.
To resolve the problems of conventional organic EL devices, use of a half mirror formed by alternatively depositing high refractive index layers and low refractive index layers, which is based on the concept of an optical microcavity, to adjust reflectivity.
FIG. 1A is a sectional view of a conventional organic EL device, and FIG. 1B is an enlarged sectional view of a half mirror.
Referring to FIG. 1A, a conventional organic EL device may include a half mirror 12 deposited on a silicon oxide substrate 11 as a stack of multiple layers, an indium tin oxide (ITO) anode layer 13, a triphenyl diamine derivative (TAD) layer 14, an ALQ (Trix-(8-hydroxyquinoline) aluminum) layer 15, and a metal anode layer 16 having a predetermined pattern, which are sequentially stacked upon one another. Here, ITO has a refractive index of 1.7–2.1, and ALQ and TAD have a refractive index of 1.7.
Referring to FIG. 1B, the half mirror 12 includes a total of six layers, including alternating large refractive index TiO2 layers 12a having a refractive index of 2.3, and low refractive index SIO2 layers 12b having a refractive index of 1.4.
In such a half mirror having the stacked structure, there is a need to stack more layers for improved reflection characteristics. However, the organic EL device suggested by the prior art requires accurate control regarding the number and the thickness of layers to be stacked to control the reflectance of light having a particular wavelength, which complicates the manufacturing process.